1. Field of the Invention
The invention relates in general to a method of fabricating nitride read-only memory (NROM) cells, and more particularly to the method of fabricating discrete NROM cell by self-aligned process.
2. Description of the Related Art
The memory devices for non-volatile storage of information, such as read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), and other advanced memory devices, are currently used in the worldwide industries. The other advanced memory devices that involve more complex processing and testing procedures include electrically erasable programmable read only memory (EEPROM), flash EEPROM, and nitride read-only memory (NROM). These advanced memory devices can accomplish the tasks that ROM can""t do. For example, using EEPROM devices in circuitry permits in-circuit erasing and reprogramming of the device.
The main characteristic of NROM is dual bit cells having multiple threshold voltage levels, where every two threshold voltage levels together store a different bit. Others store one bit on either side of the cell. The conventional structures and fabricating methods of NROM cell are described in a lot of articles and references. For example, a dual bit cell of NROM cell is disclosed in U.S. Pat. No. 6,215,148, entitled xe2x80x9cNROM cell with improved programming, erasing and cyclingxe2x80x9d. The process of NROM cell is disclosed in U.S. Pat. No. 6,201,282, entitled xe2x80x9cTwo bit ROM cell and process for producing samexe2x80x9d. The disclosure of the above-identified patent is incorporated herein by reference.
FIG. 1 is a cross-sectional view of conventional NROM cells. The substrate 10 is implanted with a source 12 and a drain 14. The top of the substrate 10 includes a sandwiched structure having a nitride layer 17 between a top oxide layer 16 and a bottom oxide layer (tunneling oxide layer) 18, the so-called xe2x80x9cONOxe2x80x9d structure. A number of BD (buried diffusion) oxide portions 20 are formed to isolate the adjacent ONO structure and form the channels 22. The structure of an NROM cell, which contains dual bits in one cell, is also depicted in FIG. 1. The larger region (encircled with the dashed line) denotes an NROM cell 30, and the two smaller regions encircled with the dashed line denote the first bit 32 and the second bit 34.
In the NROM cell, the nitride layer 17 provides the charge retention mechanism for programming the memory cell. Under normal condition, the electrons are introduced into the nitride layer 17 during a reading of the cell, while the holes are introduced into the nitride layer 17 to replace the electrons during an un-reading of the cell. Simply stated, the electrons should be able to move between the nitride layer 17 and the implant. However, nitride tends to trap electrons that are introduced in the nitride layer 17 due to its structure. If the electrons are trapped and un-removable, the cell programming function fades out or the whole cell is even ruined.
Additionally, according to the hot electron injection phenomenon, some hot electrons will penetrate through the bottom oxide layer 18, especially when it is thin, and then collected in the nitride layer 17. Concentrated charge caused by the hot electrons significantly raises the threshold of the portion of the channel 22 under charge to be higher than the threshold of the remaining portion of the channel 22. When the cell is programmed, the concentrated charge is presented and the raised threshold does not permit the cell to reach the conductive state. In a normal state, which the concentrated charge is not presented, the reading voltage over the channel can overcome the threshold of the channel 22 and consequently the channel 22 is conductive.
Moreover, the conventional NROM cell is fabricated by using several steps of photolithography. The implant and the bits are not easily formed at the right position could be shifted, so that the efficiency of the NROM cell is decreased.
It is therefore an object of the invention to provide a method of fabricating discrete NROM cell by self-aligned process, so that the problem of electrons trapped in the nitride layer of the NROM cell can be solved, and also the mutual positions of the implant and ONO structure can be exactly controlled.
The invention achieves the above-identified objects by providing a method of fabricating a discrete NROM cell by a self aligned process, comprising the steps of providing a substrate with already formed an ONO layer, wherein the ONO layer includes a top oxide layer, a nitride layer and a bottom oxide layer; etching the top oxide layer to form a discrete top oxide layer; forming a plurality of disposable spacers at the sidewalls of the discrete top oxide layer; implanting buried bit line and pocket implant by the self-aligned process; removing the discrete top oxide layer and etching the nitride layer according to the disposable spacers; removing the disposable spacers to form a plurality of discrete nitrides; etching the bottom oxide layer according to the discrete nitride layer to form a plurality of discrete pillars, so that a channel is formed between two discrete pillars; forming a plurality of channel oxides in the channels; and forming an oxide layer over the discrete channel oxides and the discrete nitrides.